Spontaneous Proton Chemistry Enables Ultralow‐temperature and Long‐life Aqueous Copper Metal Batteries

Aqueous copper metal batteries with acidic electrolytes are regarded as promising candidates for low-temperature energy storage, benefiting from fast kinetics of protons and acid resistance of copper. Here, a Cu(BF4 )2 electrolyte that spontaneously generates protons is developed for ultralow-temperature copper metal batteries. Systematic studies demonstrate that the hydrolysis of BF4- generates more protons, rendering the Cu(BF4 )2 among the most effective aqueous electrolyte capable of breaking hydrogen bonds in water molecules. This electrolyte endows a polyaniline/Cu battery to deliver a short charging time of 21 s and a charge/discharge capability of up to 10 A g-1 at -30 °C, along with a high discharge specific capacity of 70 mAh g-1 and a supercapacitor-comparable power density of 3000 W kg-1 . Furthermore, it can exhibit a long and stable cycling lifespan over 10 000 cycles at -50 °C and works well at -70 °C. This work provides an opportunity for intrinsically acidic electrolytes.